The present invention relates to a flight for collecting sludge by sweeping, which is used in a sludge-collecting apparatus operated in sewage treatment facilities, etc., and a roller shoe for such a flight.
As shown in FIG. 7, the sludge-collecting apparatus comprises a plurality of sprocket wheels 12a, 12b, . . . disposed on side walls of a sedimentation pond 11 on both upstream and downstream sides thereof, and a pair of endless chains 13 each movable around the sprocket wheels 12a, 12b, . . . along a bottom and a surface of the sedimentation pond 11. Each endless chain 13 is provided with flights 14 at an equal interval. A first metal rail 15 is disposed on the bottom of the sedimentation pond 11 between the sprocket wheels 12a, 12a and a second metal rail 16 is disposed near the surface of the sedimentation pond 11 between the sprocket wheels 12b, 12b. One of the sprocket wheels 12b rotatably engages a sprocket wheel connected to a driving means (not shown) via a chain.
Each flight 14 attached to the endless chain 13 slides along the rail 15, 16. As shown in FIG. 8, the flight 14 is provided with shoes 17, 18 which slide along the rails 15, 16 to support the flight 14. While moving along the bottom of the sedimentation pond 11, each flight 14 collects the sludge sedimented at the bottom of the sedimentation pond 11.
However, since most sewage is corrosive and contains sands, metal wastes, hard particles, etc., the shoes 17, 18 are likely to be corroded and abraded by contact with the rails 15, 17. Therefore, the flights have a short service life.
To overcome the above problem, Japanese Patent Laid-Open No. 59-162912 discloses, as shown in FIGS. 5 and 6, a flight for collecting sludge by sweeping which comprises a roller shoe 5 constituted by a U-shaped frame 51, a shaft 53 fixed to the U-shaped frame 51 by a nut 54 and a roller 52 rotatably supported by the shaft 53. The roller shoe 5 is mounted to the flight by threading means. In this flight, the roller 52 is made of a synthetic resin. With respect to the U-shaped frame 51, this reference is silent about its material, but it is usually made of stainless steel.
Recently, the materials of the flight are changing from wood to synthetic resins such as fiber-reinforced plastics (FRP). The flight made of FRP usually has a substantially F-shaped cross section or a substantially U-shaped cross section to reduce its weight. The roller shoe is mounted to such flight at its notch, and fixed together by threading means.
However, since conventional roller shoes are made of stainless steel, a separate means made of stainless steel for fixing a roller shaft by threading should be welded to a U-shaped frame. As a result, a shaft-fixing means projects outward at a substantial height. Accordingly, the notch of the flight should also be large enough to accommodate the projected portion of the U-shaped frame, resulting in a large amount of uncollected sludge remaining after sweeping.